Polyolefin plastic materials have been used to form plastic articles, especially polyolefin materials of the polyethylene type, that is, high density, medium density and low density materials. One of the difficulties which has been encountered with polyethylene plastic parts, especially those of complex shapes, is the relatively low flexural strength of polyethylene.
As is known, polyethylene is a partially crystalline and partially amorphous material, the side chain branching of the molecule being the factor which controls the degree of crystallinity. High density polyethylene has fewer side chains than low density polyethylene and accordingly a higher degree of crystallinity may be obtained. In general, increasing the crystallinity increases rigidity, tensile strength and hardness, and high molecular weight polyethylenes (those of low melt index) generally have better physical properties than the low molecular weight counterparts. Typical of the better physical properties are those such as impact resistance and stress crack resistance. However, the higher melt viscosity and the low melt index of the high molecular weight polyethylenes render them more difficult to process.
Low density polyethylenes generally are considered to have a density in the range of 0.90 to 0.925, while the high density material is generally regarded as having a density in the range of 0.941 to 0.965. For medium density polyethylene, the range is 0.926 to 0.940.
A particular material in the polyolefin family which deserves special comment is a material known as a cross-linked polyethylene which is thermosetting in its nature. In processing this type of material, a peroxide type cross-linking agent is generally used to affect the three dimensional branching network characteristic of cross-linked polyethylene.
In the case of large complex shapes used as structural members, it is frequently not possible to obtain all of the desirable physical characteristics by the use of a single polyolefin material. The difficulty which is encountered is that the use of two different types of polyethylene materials may provide the desired physical characteristics but there is considerable difficulty in providing a structural member formed of two separate plastic materials which has sufficient integrity to provide the overall desired physical characteristics, or in the alternative, the processing thereof becomes quite complicated and expensive.
By way of example, a cross-linked polyethylene material has a relatively low flexural modulus, about 100,000 psi. Normally, the processing includes starting with a high density material which, when cross-linked lowers the density of the material. For example one may start with a polyethylene having a density of 0.955 and when cross-linked the density is approximately 0.94 which is in the range of the lower end of the high density materials or the higher end of the medium density materials, or what is sometimes referred to as a medium density material.
Where high flexural strengths are needed, for example in structural components for the recreational market such as recreational vehicles and boats, the use of a cross-linked polyethylene material does not provide sufficient flexural strength.
It is also known that high density polyethylene foams are quite rigid, having a flexural modulus of between 200,000 and 300,000 psi. These particular materials, however, have a relatively low impact resistance in that a foamed product may be easily fractured. Thus, in those instances in which the plastic part is to be subjected to impact, for example, automotive doors and tops, camper tops, boats, carboys and containers, the use of a polyethylene foam which has an appreciable flexural modulus presents practical problems.
It is possible, separately to form two components and join them together by a bonding procedure. By way of example, polyethylene and polypropylene may be heat sealed, but generally require melting. Where a particular part is relatively large in size, such a boat, or a camper top or a top for a recreational vehicle, joining together two separately formed plastic elements by an adhesive or by any of the conventional bonding methods used in the plastics industry is not acceptable from the standpoint of the result produced and the substantial expense necessary to handle large bulky items of complex shapes.
Nonetheless, the potential strengths obtained from polyolefin plastic materials, and perhaps other thermal plastic materials as well as thermosetting materials renders them attractive candidates for the formation of structural parts provided the structural part has sufficient integrity to remain tightly adhered such that the strengths of the resulting product are sufficiently high for the intended use. Accordingly, it is desirable to provide a plastic article having the desirable features of rigidity, thermal and sound insulation, flotation, impact resistance, relative lightweight, and weatherability. Moreover, it is desirable to be able to fabricate complex shapes so as to provide a bond between the respective components of the plastic article which assures the maximum utilization of the strengths of the individual components making up the plastic article. Particularly advantageous is a method by which an article may be formed in one operation such that separate processing of separate parts followed by a step of adhering the two together is eliminated.
Typical of the prior art patents are U.S. Pat. Nos. 3,649,407; 3,748,214; 3,655,497; 3,673,033; 3,705,071; 3,458,380; 3,472,715; 3,715,256; 2,341,260; 3,709,966; 3,607,600; 3,193,437; 3,228,819; and 3,709,967.